Relative Role of Overwash and Aeolian Processes on Washover Fans, Assateague Island, Virginia-Maryland

Abstract

There has been considerable controversy in recent years about the relative importance of overwash vs. aeolian processes in the vertical accretion or erosion of barrier islands. Most studies have used sediment budget data sets of two years or less in length. Analysis of coastal climatic data suggests that there are significant secular variations in storm intensity, frequency, and duration over time scales of 10's of years. Annual variations in climatic variables such as the magnitude and frequency of precipitation, high winds, and overwash have a considerable effect on accretion or erosion of washover fan surfaces along mid-Atlantic barrier islands. Conclusions and predictions based on data sets from very stormy years may he dramatically different than those based on data from relatively calm periods. Sediment budget data for six fans along Assateague Island are presented for a four-year period that includes an exceptionally stormy year, an exceptionally calm year, and two years having average storm history. Accretion due to overwash processes was exceedingly dominant during the stormy year, but still significant in normal years. In contrast, aeolian deflation and deposition dominated during the calm year. Overall, Assateague Island fan surfaces accreted significantly over the course of the study, by an island-wide average of 22 cm. Individual fans accreted by as much as 90 cm. Observed variations in fan surface changes depended primarily upon the frequency of precipitation, frequency of overwash, and topography of the local fan area. The importance of overwash or aeolian processes with regard to fan accretion and erosion appears to depend upon variability and cyclicity of climatic factors. The effects of two exceedingly large storms resulted in extensive accretion on fan surfaces that was greater than the sum of 15 moderate storms during the four years. Longer observations periods may be needed to project future changes on barrier island surfaces.